Back to EveryPatent.com
| United States Patent |
5,119,303
|
|
Struck
, et al.
|
June 2, 1992
|
Brake-force control system for vehicles
Abstract
Brake-force control system (ABS) for vehicles, in particular motor
vehicles, with separate control of the front wheel brakes and with joint
or separate rear wheel brakes which can be switched over to "select high"
control or individual control and/or with a limited-slip differential at
the rear axle, the switchover of the rear wheel brakes and/or of the
limited-slip differential being effected at different coefficients of
friction (.mu.-split) with a specifiable rate of rise dependent on the
travelling speed, and in which, in order to increase directional stability
during a braking procedure, a switching device is provided which blocks a
switchover of the rear wheel brakes and/or of the limited-slip
differential until the travelling speed from the time of the detection of
different coefficients of friction has decreased to a specified value, the
rate of rise of the switchover being dependent at least one the duration
of the reduction of the vehicle speed to the specified value.
| Inventors:
|
Struck; Helmut (Winnenden, DE);
Fischle; Gerhard (Esslingen, DE);
Lehmann; Bernhard (Appenweier, DE)
|
| Assignee:
|
Daimler-Benz AG (DE)
|
| Appl. No.:
|
454858 |
| Filed:
|
December 22, 1989 |
Foreign Application Priority Data
| Current U.S. Class: |
701/73; 303/149 |
| Intern'l Class: |
B60T 008/32 |
| Field of Search: |
364/426.02,426.03,424.05
303/96-100
180/197
|
References Cited
U.S. Patent Documents
| 4759589 | Jul., 1988 | Leiber | 364/426.
|
| 4759590 | Jul., 1988 | Uchida et al. | 364/426.
|
| 4770266 | Sep., 1988 | Yamaguchi et al. | 364/426.
|
| 4793662 | Dec., 1988 | Arikawa | 364/426.
|
| 4837694 | Jun., 1989 | Narita et al. | 364/426.
|
| 4852009 | Jul., 1989 | Jonner et al. | 364/426.
|
| Foreign Patent Documents |
| 2518196 | Nov., 1976 | DE.
| |
| 3421700 | Dec., 1984 | DE.
| |
| 3627550 | Feb., 1988 | DE.
| |
Primary Examiner: Lall; Parshotam S.
Assistant Examiner: Queen; Tyrone
Attorney, Agent or Firm: Evenson, Wands, Edwards, Lenahan & McKeown
Claims
What is claimed:
1. An anti-lock vehicle braking system for a motor vehicle comprising:
an anti-wheel lock means for separately controlling each front wheel of the
vehicle and for subjecting rear wheels of the vehicle to one of joint and
separate control; and
a switching means for switching between the joint and separate control of
the rear wheels when different coefficients of friction between left-hand
and right-hand wheels of the vehicle are present, operation of the
switching means being effected at a specifiable rate of rise of brake
pressure dependent on a vehicle speed signal,
wherein the switching means delays the switchover of the rear wheels until
the vehicle speed signal has decreased from a value at a time of the
detection of the different coefficients of friction to a specified value;
and
wherein the rate of rise is dependent at least on a duration of the fall of
the vehicle speed signal to the specified value.
2. An anti-lock vehicle braking system according to claim 1, wherein the
specifiable value of the vehicle speed is determined by multiplication of
the vehicle speed, measured at the time of the detection of different
coefficients of friction, by a factor which is dependent on the vehicle
speed measured at the time of detection of different coefficients of
friction.
3. An anti-lock vehicle braking system according to claim 2, wherein the
factor is additionally dependent on a difference in the vehicle
deceleration between a time at a beginning of control and the time of the
detection of different coefficients of friction.
4. An anti-lock vehicle braking system according to claim 2, wherein the
factor is additionally dependent on the length of the time between the
beginning of control and the time of the detection of different
coefficients of friction as a function of the vehicle speed.
5. An anti-lock vehicle braking system according to claim 2, wherein the
factor is additionally dependent on one of an instantaneously travelled
bend radius and on parameters associated with the latter.
6. An anti-lock vehicle braking system according to claim 1, wherein the
rate of rise is additionally dependent on a braking deceleration achieved
at a switching time of the switching means.
7. An anti-lock vehicle braking system according to claim 1, wherein the
rate of rise is additionally dependent on one of a bend radius being
travelled at a switching time of the switching device and on parameters
associated with said bend radius.
8. An anti-lock vehicle braking system according to claim 1, wherein the
duration is additionally dependent on reaction and disturbance-correction
times obtained from a series of tests with ordinary drivers.
9. An anti-lock vehicle braking system according to claim 1, futher
comprising a limited-slip differential which is switched on for connecting
together the rear wheels of the vehicle, wherein the switching off means
further controls the switching of the limited-slip differential when
different coefficients of friction are present.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention generally relates to an anti-lock vehicle braking
system (ABS), and more particularly, to such a system in which better
stability of the vehicle is achieved during a braking procedure.
A vehicle braking system of the generic type, with an anti-lock system
(ABS), is disclosed by German Published, Unexamined Patent Application
3,627,550. In this system, in the presence of different coefficients of
friction (.mu.-split) during an ABS-control procedure to shorten the
braking distance, in the case of a four-channel ABS, the braking control
method for the rear wheels is switched over from "select low" to "select
high". In the case of a three-channel ABS, the response/slip threshold for
the beginning of control of the rear wheel brakes is raised and in the
case of a vehicle with ABS and an automatically slip-limitable
differential (ASD) this differential is blocked. In order to avoid
provoking a sudden yawing reaction of the vehicle due to this switchover,
provision is made, at least at higher vehicle speeds, to permit the rise
of the brake pressure at the wheel with the higher adhesion coefficient or
the raising of the slip threshold or the rise in the blocking effect of
the ASD only at a particular specifiable rate of rise.
A vehicle braking system with an anti-lock system has furthermore been
disclosed by German Published, Unexamined Patent Application 3,421,700, in
which, to avoid high yawing moments in the case of asymmetrical
coefficient of friction of the roadway and overlong braking distances, the
brakes on the vehicle side having the higher coefficient of friction can
be controlled as a function of the brakes on the side having the lower
coefficient of friction with a time delay to produce a clocked increase of
the brake pressure.
An anti-lock system for vehicles is furthermore disclosed by German
Published, Unexamined Patent Application 2,518,196, in which the control
can be switched over between the operating modes "select low" and "select
high" as a function of various conditions, a switchover to "select high"
operation being prevented for a period specified by a timing element when
certain criteria are present.
It is an object of the invention to provide an anti-lock vehicle braking
system of the generic type with measures for shortening the braking
distance under certain conditions in such a way that even better stability
of the vehicle during the braking procedure is achieved.
Compared to anti-lock vehicle braking systems of the generic type discussed
above, particular advantages are obtained by preferred embodiments of the
invention which includes that, at the beginning of an ABS control
procedure, the driver is first given the opportunity of correcting any
yawing reaction which occurs and that a higher brake-force is switched in
only after this so-called stabilization phase. Any yawing reaction which
occurs can then be corrected again, with the result that overall a more
stable ride in combination with shortening of the braking distance is
achieved.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic representation of an anti-lock vehicle braking
system;
FIG. 2 shows a flow chart for the control of the anti-lock vehicle braking
system;
FIG. 2a shows an associated routine for the control of the anti-lock
vehicle braking system; and
FIG. 3 shows a time-dependency diagram.
DETAILED DESCRIPTION OF THE DRAWINGS
An exemplary embodiment of the invention is illustrated in the drawings and
is described below in greater detail.
The schematic representation in FIG. 1 shows a module designated ABS which
contains the entire electronic system of the ABS for a vehicle and which
is supplied with the signals N.sub.R generated by wheel speed sensors (not
shown) and a signal B generated when a brake pedal (not shown) is
actuated.
From these signals, the electronic system generates control signals in a
manner known per se for the schematically represented, electromagnetically
actuable control valves (3-way valves) FL, FR, RL, RR (front left, front
right, rear left, rear right) for raising, lowering and holding constant a
brake pressure at individual wheels of the vehicle.
Brake pressure control at the front wheels is here effected individually in
known manner as a function of individual rotational behavior, and control
of the rear wheels is effected according to the so-called "select low"
principle, i.e. as a function of rotational behavior of the wheel with the
smaller adhesion coefficient.
A switching device 1, according to a preferred embodiment of the invention,
is contained, for example, in one single module. This switching device 1
is supplied, as input quantities, with a vehicle speed V.sub.F,and a
braking signal B which for the sake of simplicity is taken from the ABS
since it is already present there. Also provided to the switching device 1
are a control signal S.sub.R, which appears when a control procedure is
taking place in the ABS, a detection signal S.mu., which appears when, by
reason of the wheel speeds, different coefficients of friction are
detected during a control procedure of the ABS, and a bend signal S.sub.K,
which contains information on the bend radius being travelled. The bend
signal S.sub.K, for example, is derived from a transverse acceleration
transmitter, steering angle transmitter or yaw angle transmitter.
The output signal of the switching device 1 is supplied to the ABS
electronic system as, for example, an analog switching signal S.sub.S and
there effects a switchover of the rear wheel braking control method from
"select low" to "select high" or provides individual control with a
corresponding rate of rise of the brake pressure.
If, in addition to the ABS, the vehicle also has an automatically
slip-limitable differential (ASD), which is usually switched off
immediately when the ABS responds, the switching signal S.sub.S can also
be used, in the case of a controlled braking, to block this ASD with a
specified rate of rise. This possibility is likewise represented
schematically in FIG. 1, wherein the control signal S.sub.R is
additionally supplied to a switching element 2 of the ASD, by control of
which, the switching signal S.sub.S can be released.
The functioning of the braking control system, in particular also of the
switching device 1, is now described in greater detail in conjunction with
FIGS. 2 and 3.
As the flow chart in FIG. 2 shows, a "monitoring routing for monitoring the
wheel speeds N.sub.R " runs in a program step 10 in the ABS module. If the
driver now actuates the brake, i.e. brake signal B in FIG. 1 and program
step 11, at time t.sub.0 (See FIG. 3) then, in accordance with program
step 12, a new monitoring routing is started in the ABS module. If the ABS
detects the presence of different wheel speeds, then, in a program step
13, at time t.sub.01, ABS control starts. At same time, t.sub.02, in a
program step 14, the ABS detects whether, on the basis of the wheel
speeds, .mu.-homogeneous or .mu.-split is present.
If .mu.-homogeneous is present, then, in accordance with program step 15,
the ABS control is carried out in accordance with the select-low principle
at the rear axle. In a program step 16, a test is then carried out to
determine whether ABS control is still continuing. If yes, the sequence is
set back to program step 14 and if no, a test is carried out in a program
step 17 to determine whether the brake is still being actuated, that is,
whether signal B is still present. If not, the sequence is set back to
program step 10 and if yes, the sequence is set back to program step 12.
If necessary, a new run is started.
However, if .mu.-split is detected in program step 14, i.e. time t.sub.02,
the switching device 1 is correspondingly activated by the signal
S.sub..mu. in which switching device, in a program step 18, in accordance
with a routine 18.1, a factor k and, in a routine 18.2, the time
.DELTA.t.sub.23 is determined (See FIG. 2a).
The factor k is, for example, read from a characteristic diagram memory
18.1.1 using the signal quantities v.sub.02 and S.sub.K at time t.sub.02,
the vehicle deceleration .DELTA.b.sub.12 in the period .DELTA.t.sub.12,
and the time .DELTA.t.sub.12. In the routine 18.2, the product
(k.v.sub.02) and thus the value v.sub.03 is then specified, the factor
being of the order of, for example, 0.85, and the duration .DELTA.t.sub.23
is then calculated. It is possible, in the context of a plausibility
check, for the duration .DELTA.t.sub.23 to be corrected by reaction and
disturbance-correction times t.sub.DCO obtained from a series of tests
with ordinary drivers.
If the vehicle speed has now decreased to the specified value v.sub.03 and
if, in this time, no .mu. jump has occurred, routine 18.4, then, in
routine 18.3, using the duration .DELTA.t.sub.23, the instantaneously
attained braking deceleration b.sub.03 and the quantity S.sub.K, the rate
of rise .alpha. of the switchover of the rear wheel brakes and/or of the
limited-slip differential is determined. This is initiated at switching
time t.sub.03 by the switching signal S.sub.S if it is established, in
accordance with a program step 19, that the ABS control procedure is still
continuing. If, on the other hand, the control procedure is already at an
end, then, in accordance with a program step 20, the switching signal
S.sub.S is suppressed, i.e. no switchover is initiated, and the program is
advanced to program step 17.
If, however, a .mu. jump has occurred in the time in which the vehicle
speed has decreased to the value v.sub.03, then, in routine 18.5, a test
is carried out to determine whether the jump in the case of .mu.-split is
in the same sense. If this is the case, the program is advanced to routine
18.3, and in this routing the rate of rise is adjusted to the new
.mu.-split condition. If this is not the case, then, in a program step 21,
the switching signal S.sub.S is suppressed and the program is advanced to
program step 16, in which a test is then carried out to determine whether
the ABS control is still continuing. If yes, the sequence is set back
again to program step 14 and continued until the ABS ceases to control. If
ABS control is at an end, then, in program step 17, a test is carried out
to determine whether the brake is still being actuated. If braking is not
yet at an end, the sequence is set back to program step 12, while, if
braking is at an end, the program run is also at an end and the program is
switched back to program step 10.
This braking control system thus gives the driver the opportunity of
correcting a possible yawing reaction occurring at the beginning of an ABS
control procedure even before the rear wheel brakes or the limited-slip
differential is switched over, since the switchover occurs only after a
time delay, i.e. after a stabilization phase.
Although the present invention has been described and illustrated in
detail, it is to be clearly understood that the same is by way of
illustration and example only, and is not to be taken by way of
limitation. The spirit and scope of the present invention are to be
limited only by the terms of the appended claims.
Top